Biomedical subjects

K J Ciuffreda

Publications and source records attributed to K J Ciuffreda.

At least 37 records · Page 2Linked to original sources

PubMed1995

Do changes in pupil size and ambient illumination affect the duochrome test?

Vague and conflicting suggestions currently exist within the literature regarding the appropriate level of ambient illumination to use while conducting the clinical duochrome (bichrome) test. One proposal is that reduced illumination should be adopted since this allows the pupils to dilate, thereby increasing the chromatic interval and resultant test sensitivity. The aim of the present study was to determine whether changes in pupil size and ambient illumination do indeed alter the subjective duochrome response. Accordingly, eight young, visually normal subjects were dilated with 2.5 percent phenylephrine and tested while monocularly viewing the duochrome chart through 1, 3, 5 and 7mm artificial pupils. Additionally, the effects of performing the test through a 1.0 log unit neutral density filter and varying the overall room illumination were examined. There were no significant differences between the mean responses under all test conditions. The variability of individual responses decreased with increasing pupil size; however, a 4mm change in pupil diameter was required to alter the response variability significantly. Therefore it is recommended that the test be performed under the minimum practical ambient illumination conditions in order to minimize this variability.

Accommodation, Ocular↗

PubMed1995

Effect of age on the interaction between the AC/A and CA/C ratios.

A number of previous studies have examined the effects of age and the development of presbyopia on either the accommodative convergence to accommodation (AC/A) or convergent accommodation to convergence (CA/C) ratios. However, changes in the interaction of these oculomotor crosslinks with increasing age have received relatively little attention, especially over a wide age distribution. Accordingly, the present study examined both crosslink ratios in 42 subjects ranging from 22 to 65 years of age. A dual haploscope-optometer was used to measure accommodation and vergence. The response AC/A ratio showed a small but significant positive correlation with age in those subjects under 45 years of age, while no significant correlation was observed between the stimulus AC/A ratio and age for all subjects. The CA/C ratio exhibited a significant negative correlation with age for all subjects. Furthermore, a trend was observed for lower CA/C findings to be associated with higher response AC/A ratios. However, the two ratios were inversely but not reciprocally related, with the mean difference between the CA/C ratio and the reciprocal of the response AC/A ratio being significantly different from zero. These age-related changes in the output of the oculomotor crosslinks provide further evidence of the continual oculomotor adaptation that occurs concurrent with the loss of accommodative responsivity.

Accommodation, Ocular↗

PubMed1995

Effect of target proximity on transient myopia induced by equidioptric stimuli.

Transient myopia may be exhibited after sustained focus on a near target. This appears to be related to the within-task accommodative response. Although proximally induced accommodation has been shown to produce considerable transient myopia under open-loop conditions, its effect under naturalistic closed-loop conditions remains unclear. Accordingly, the present study examined the effect of target distance on transient myopia for equidioptric stimuli (5 D). A Canon R-1 autorefractor was used to assess the pre- and post-task refractive state objectively. Two tasks were performed monocularly and consisted of either accommodating on a near target at 20 cm (5 D) or a far target at 6 m viewed through a -5 D lens. The former task involved both blur and proximally induced accommodation, whereas the latter consisted of a blur stimulus only. Similar amounts of transient myopia were found under both conditions. These equivalent responses may be related to the relatively small output of proximally induced accommodation believed to be present under closed-loop conditions.

Accommodation, Ocular↗

PubMed1995

Abnormal transient myopia in symptomatic individuals after sustained nearwork.

After a sustained period of nearwork, some patients report slightly blurred distance vision for several seconds or even minutes. This presumed large and prolonged "abnormal transient myopia" has never been objectively documented. A Canon R-1 infrared autorefractor was used to assess distance refractive state every 2 s before and immediately after a binocular near task in three such individuals. Results showed a large post-task myopic shift, slowed baseline decay, and increased response variability as compared with that found in asymptomatic visually normal individuals. These findings suggested that this transient distance blur was related to the increased transient myopia resulting from one or more possible abnormalities related to the accommodative system.

Accommodation, Ocular↗

PubMed1994

Disparity vergence eye movements exhibit preprogrammed motor control.

To determine the control strategy used to mediate disparity vergence eye movements, an optical technique was employed which eliminated the visual feedback normally associated with a vergence movement. The resulting "open-loop" vergence responses from four subjects exhibited multiple step-like movements. This finding augments previous demonstrations of multiple-step tracking strategies in vergence eye movements and supports the theory that the initial portion of the vergence response is under preprogrammed control. Additional evidence for preprogramming is given by the similarity of the initial dynamics between open- and closed-loop responses as evidenced by similar ratios of peak velocity to amplitude (i.e. "main sequence" plots).

Convergence, Ocular↗

PubMed1994

Accommodative adaptation in children.

Although accommodative adaptation has been studied extensively in young adult populations, there has been little consideration of this function in children. Accordingly, we examined accommodative adaptation by comparing pre- and post-task measurements of dark accommodation (DA) in children and young adults. DA was assessed objectively before and immediately after a 5-min near-vision task (viewing distance = 20 cm) using an open-field, infrared optometer (Canon Autoref R-1). The study was performed on 15 visually normal children (mean age = 10.1 years) and 10 visually normal young adults (mean age = 25.2 years). No significant difference was observed between the mean values of pre-task DA for the two age groups (children = 1.01 D, adults = 0.88 D). Both groups showed equivalent degrees of accommodative adaptation (approximately 1.0 D), and the rate of regression of post-task DA to the pre-task level was similar for each group. We found a significant positive correlation between the magnitude of adaptation and pre-task DA in children, which was not found in the young adult population. The results indicate that pre-task DA and accommodative adaptation (both in terms of magnitude and rate of decay) are similar in adults and children. This would not support the proposal that the development of nearwork-induced myopia in children is related to short-term variations in accommodative adaptation when compared with young adults.

Accommodation, Ocular↗

PubMed1994

Cognitive demand and transient nearwork-induced myopia.

Previous studies have suggested that nearwork-induced myopia may be related to the amount of mental effort and cognitive demand required during the course of near-vision tasks. Accordingly, we compared measurements of the far point of accommodation (FPA) obtained before and immediately after 10-min sustained near-vision tasks involving either low, moderate, or high cognitive demand. All measurements of the FPA were obtained objectively using an open-field infrared optometer. Although a significant myopic shift in the FPA was observed after all conditions, no significant difference was recorded between the three cognitive levels. The mean myopic shift in the FPA immediately after the near-vision task was 0.23 D. It is concluded that the transient, task-induced myopic shift in the FPA is related to the within-task accommodative response rather than to variations in cognitive demand during the course of the near-visual activity.

Accommodation, Ocular↗

PubMed1994

Sensitivity analysis of relative accommodation and vergence.

A sensitivity analysis was performed to determine the variation in response to changes in parameter values of a previously developed nonlinear static model of accommodation and vergence. To determine normal behavior, model simulation responses were computed using previously obtained parameter values in 4 subjects under 2 conditions. In the first, relative accommodation was evaluated by maintaining the vergence stimulus constant at 2.5 meter angles (MA) and varying the accommodative stimulus from -2.5 to 2.5 diopters (D) in 0.25-D steps. In the second, relative vergence was evaluated by maintaining the accommodative stimulus constant at 2.5 D and varying the vergence stimulus from 25 prism diopters (PD) base-in to 25 PD base-out in 5-PD steps. Sensitivity of the model parameters, consisting of controller gains for accommodation (ACG) and vergence (VCG), crosslink gains for accommodation-to-vergence (AC) and vergence-to-accommodation (CA), deadspace operators for accommodation (AE +/- AD) and vergence (VE +/- VD), and the tonic levels for accommodation (ABIAS) and vergence (VBIAS) were assessed by varying them at 50% and 150% of their normal values. It was found that the accommodation and vergence systems were most sensitive to variation in crosslink gain, moderately sensitive to variation in controller gain and tonic level, and least sensitive to variation in size of the deadspace. These results may provide a quantitative basis for the occurrence of ocular dysfunctions associated with abnormal crosslink gains, such as strabismus, in clinic patients.

Accommodation, Ocular↗

PubMed1994

Tonic accommodation: a review. II. Accommodative adaptation and clinical aspects.

Part I of this review considered basic aspects of tonic accommodation (TA), i.e. the accommodative response observed under degraded stimulus conditions. Part II considers accommodative adaptation, i.e. the apparent change in TA following periods of sustained fixation, and clinical aspects of both baseline TA and accommodative adaptation. It is suggested that the apparent post-task shift in TA reflects the slow rate of decay of the stimulus-mediated adaptive accommodative response, while the actual level of tonic innervation to the ciliary muscle remains relatively constant. The clinical implications of both TA and accommodative adaptation are discussed with regard to night, space and instrument myopia and refractive error development, notably nearwork-induced myopia. It is concluded that the evidence for any association between this form of myopia and either TA or accommodative adaptation is equivocal, and furthermore it seems likely that TA plays only a minor role in influencing the closed-loop steady-state accommodative response.

Accommodation, Ocular↗

PubMed1994

Pursuit eye movements in chronic schizophrenics: relationship between increased saccades and negative symptoms.

Smooth pursuit eye movements of chronic schizophrenics (n = 16) and controls (n = 10) were recorded while subjects tracked a sinusoidally-moving target. Negative symptoms in schizophrenics were rated using the Schedule for the Assessment of Negative Symptoms. An increased frequency of smooth pursuit eye movement abnormalities was found in the schizophrenic group, including greater variability of gain and more frequent and larger saccades. Frequency of saccades was positively correlated with the Negative Symptoms scales for attentional impairment and alogia. The results confirm findings of eye movement dysfunction in schizophrenics and provide new evidence that the motor and cognitive dysfunctions of schizophrenia may be related.

Adult↗

PubMed1994

Vergence eye movements under natural viewing conditions.

PURPOSE: To determine whether there are any fundamental differences in vergence dynamics under different viewing conditions, both in instrument space and free space. METHODS: Symmetric vergence responses were measured for a variety of conditions under the traditional instrument space as well as the more natural free space viewing environment. Vergence eye movements were recorded objectively in three subjects using the infrared reflection technique. Within each environment, four conditions were tested: aperiodic self-initiated voluntary gaze shifts between two simultaneously viewed targets; periodic (0.33 Hz) voluntary gaze shifts between the same two targets; gaze shifts in total darkness to the near target following initial far target fixation in the light; and gaze shifts in total darkness to the two remembered target positions. In addition, an experiment was performed in instrument space using randomized step changes of target disparity, in which the responses served as the standard for comparison. For all conditions, target disparities ranged from 0.5 degrees to 10 degrees. The peak velocity of each vergence response was calculated and plotted versus its amplitude. RESULTS: It was found that the data for all conditions tested fell within the standard "main sequence" cluster, indicating similarity in dynamics and thus similarity in the motoneuronal controller signal. Also, the data from investigators who claimed differences in dynamics were also typically found to fall within the normal cluster. CONCLUSIONS: This indicates that the vergence motoneuronal controller signal produced the same dynamics for a particular response amplitude, independent of both viewing environment and test condition.

Adult↗

PubMed1993

Temporal characteristics of proximally-induced accommodation.

This study sought to determine whether proximally-induced accommodation (PIA) is capable of sustaining an accommodative response over time. Accordingly, the steady-state PIA response was measured during the course of a continuous 5 min stimulus period for targets located at distances of 0.2 or 6 m. The vergence and accommodation loops were opened by subjects (n = 8) monocularly viewing the targets through a 0.5 mm pinhole, while accommodation was assessed using an objective, open-field, infrared optometer. The mean PIA response did not change significantly during the fixation period. This finding suggests that PIA is indeed capable of maintaining a sustained response. This observation is consistent with the constancy of apparent target distance perception during extended fixation of a stationary object of regard under these conditions.

Accommodation, Ocular↗

PubMed1993

Tonic accommodation: a review. I. Basic aspects.

In the absence of an adequate visual stimulus, accommodation adopts an intermediate position of approximately 1 D. Since this position was believed to reflect the level of tonic innervation to the ciliary muscle, this response has been termed tonic accommodation (TA). Part I of this review will consider various aspects of this parameter, including its reference to closed-loop accommodative function and autonomic physiology. In addition, both the methods of measurement and appropriate terminology for this function will be discussed. It is concluded that the response, which becomes apparent under so-called 'stimulus-free' conditions, in fact probably represents an aggregate response resulting from multiple, non-optical stimuli. Thus the designation tonic accommodation may not be appropriate, since it fails to describe accurately the heterogeneous composition of the stimulus-free accommodative response. An associated paper (to be published as part II of this review) will examine accommodative adaptation and both clinical aspects of TA and adaptation of TA.

Accommodation, Ocular↗

PubMed1993

Tonic vergence, age and clinical presbyopia.

The relative contribution of tonic vergence to the maximum near vergence response as a function of age and the development of presbyopia has received little attention. Tonic vergence position was measured subjectively in a group of 50 subjects between 20 and 68 years of age. While there was no progressive trend with age, tonic vergence did exhibit a small but consistent convergent increase after 40 years of age. Thus, tonic vergence showed a slight change, possibly of a motor adaptive nature, concurrent with increasing age and the development of clinical presbyopia.

Adult↗

PubMed1993

Static accommodation in congenital nystagmus.

PURPOSE: To conduct a comparative study of static accommodative function between individuals with normal vision (n = 10) and patients with congenital nystagmus (n = 12). METHODS: The component contribution to monocular steady-state accommodation (slope of the accommodative stimulus/response function, accommodative controller gain, tonic accommodation, and depth-of-focus) was assessed subjectively using a Hartinger coincidence-optometer, except for depth-of-focus, which was determined psychophysically. RESULTS: The group mean slope for the patients with nystagmus was not significantly different from that found in the normal subjects. However, their variability was markedly increased. Therefore, the patients with nystagmus were divided into three subgroups with regard to the normal accommodative stimulus/response function slope criterion. The majority of patients with nystagmus (n = 10) exhibited slopes that were outside of normal limits, being greater than (n = 4) or less than (n = 6) the normal range. Depth-of-focus was the only parameter found to be significantly different between the normal and the nystagmus groups. When the nystagmus group was divided with respect to etiology--ie, albinotic (n = 4) versus idiopathic (n = 8)--there were no significant differences found for the various accommodative parameters. CONCLUSIONS: We speculate that the primary component contributing to the anomalous accommodative behavior was the increased depth-of-focus, with this perhaps being related to abnormal fixational eye movements and eccentric fixation, and more generally related to overall reduced sensitivity resulting from the early abnormal visual experience.

Accommodation, Ocular↗

PubMed1992

Factors influencing accommodative adaptation.

Previous studies have ascertained that a broad range of parameters have the facility to induce or modify accommodative adaptation, i.e., the post-task shift in dark accommodation (DA) which may be observed after a period of sustained fixation. This paper reviews the optical (i.e., related to a change in the form of the retinal image) and nonoptical (i.e., independent of the composition of the retinal image) stimuli which can influence the degree of adaptation. It is concluded that DA probably represents an aggregate response resulting from multiple inputs whose interactions may be too complex to allow the isolation and measurement of any single nonoptical component. Accordingly, the exact mechanism of accommodative adaptation remains unclear at the present time.

Accommodation, Ocular↗

PubMed1992

Symptoms related to abnormal tonic state: experimental results and computer simulations.

There have been numerous attempts to relate tonic accommodation and tonic vergence and their adaptive capabilities to a variety of parameters in vision science and clinical optometry. In this paper, investigations attempting to correlate the tonic state with nearwork symptoms will be reviewed. Results suggest that tonic vergence is more closely linked to near symptoms than is tonic accommodation. The results will be discussed in terms of a steady-state model of accommodation and vergence.

Accommodation, Ocular↗

PubMed1992

Contribution of accommodation and disparity-vergence to transient nearwork-induced myopic shifts.

Many investigations have attempted to demonstrate a relationship between sustained near-vision and the aetiology of environmentally-induced myopia, but it remains unclear whether myopic development relates to the actions of either accommodation or vergence. The present study investigated the effect of varying the disparity-vergence demand during a near-vision task while maintaining the accommodative stimulus constant. We required 20 young subjects to perform a near task (viewing distance = 20 cm) for 20 min through base-in, zero power and base-out prisms. The magnitude of the prisms for each individual corresponded to one-third their near vergence range. Pre- and post-task measurements of refractive state were determined objectively using an infrared optometer, with post-task readings being recorded over the initial 50 s following task completion. The results indicated no significant difference between the transient far-point shift for the three disparity-vergence conditions. However, combining data across conditions, a significant, mean shift in the myopic direction of 0.14 D was observed during the 10-20 s period immediately following task completion which dissipated within 20-50 s post-task. These findings suggest that the transient post-task myopic shift was not related to the output of disparity-vergence.

Accommodation, Ocular↗